1. Field of the Invention
The present invention refers to a full-flow rocket nozzle having a longitudinal contour essentially corresponding to a parabola.
2. Description of the Related Art
The function of the rocket nozzle is to expand and accelerate the gas to high velocity and thereby give thrust efficiency and payload capacity. The capability to expand the gas is limited by the fact that the ambient pressure at sea level forces the flow to separate. The separated flow generates unsteady aerodynamical forces. These forces set the limit for the size of the nozzle and hence the limits for nozzle efficiency in vacuum operation. For thermal reasons first stage nozzles may be run separated towards the exit. This means that if the unsteady aerodynamical forces could be reduced to an acceptable level the nozzles could be run continuously separated at the ground and in ascent. This would provide for the possibility to make larger and more effective nozzles.
Normally, the nozzle contour is a continuous smooth contour optimized to minimize performance loss in shocks and divergence of the flow. The contour is typically described by a parabolic function. Hitherto the nozzle contours have not been optimized for separated flow at sea level since no side load control measures to allow this has been known.
Attempts have been made to influence indirectly on the nozzle contour. Such prior attempts have included for example an exit diffuser means, see EP 626 513 A1, a trip ring, an abladeable or ejectable insert, periodically variable radius, see PCT/SE96/00176, slotted nozzles, see U.S. Pat. No. 5,450,720, injection of gas, see U.S. Pat. No. 4,947,644, an exit ejector means, see EP 894 031 and a mechanism for extendible exit nozzle, see EP 924 041. A flow separation control device with a modified nozzle contour is known from U.S. Pat. No. 3,394,549.
Therefore, prior full flow nozzles have a limited expansion ratio that limits the performance. Said performance is optimized considering the flow separation margin and the side loads.
All the measures mentioned above are adapted to be used in combination with a parabola or bell contour. This leads to long and heavy nozzles. The net performance gain including weight increase is moderate and is decreasing as the nozzles are made very large. The bell nozzle has, towards the exit, a flat gradient in wall pressure vs. axial length as it is optimized for performance. However, this is contradictory to optimizing for side loads. The weight of the nozzle has a negative impact on the weight and complexity of the engine system and the rocket thrust structure. The size of the nozzle may not be compatible with the space available in test facilities, in rocket assembly and at rocket launch sites.
Thus the main object of the present invention is to suggest a nozzle contour which eliminates the aforementioned drawbacks. The basic idea of the present invention is that the effective parabolic contour is kept for as long portion of the nozzle contour as where the pressure is still high and has a strong impact on the resulting performance. Thereafter, combinations of nozzle contours are suggested. The present invention thus is mainly distinguished in that the parabola contour shape from the point of 50% expansion ratio onwards or more distant from the throat of the nozzle is changing over into (i) a strictly conical shape having an angle to the center line of between 15xc2x0 and 25xc2x0, (ii) a slight outward curvature, i.e. implying a contour shape with positive 2nd derivative of the radius r, or (iii) a slight inward curvature, i.e. with a negative 2nd derivative of r but lying externally of the parabola contour, in the last case the 3rd derivative of r being constantly equal to zero (0).